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(#) Creating a MutableState object with a mutable collection type

!!! WARNING: Creating a MutableState object with a mutable collection type
   This is a warning.

Id
:   `MutableCollectionMutableState`
Summary
:   Creating a MutableState object with a mutable collection type
Severity
:   Warning
Category
:   Correctness
Platform
:   Any
Vendor
:   Jetpack Compose
Identifier
:   androidx.compose.runtime
Feedback
:   https://issuetracker.google.com/issues/new?component=612128
Min
:   Lint 8.0 and 8.1
Compiled
:   Lint 8.0 and 8.1
Artifact
:   [androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.md.html)
Since
:   1.5.0
Affects
:   Kotlin and Java files and test sources
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/main/java/androidx/compose/runtime/lint/MutableCollectionMutableStateDetector.kt)
Tests
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/MutableCollectionMutableStateDetectorTest.kt)
Copyright Year
:   2021

Writes to mutable collections inside a MutableState will not cause a
recomposition - only writes to the MutableState itself will. In most
cases you should either use a read-only collection (such as List or Map)
and assign a new instance to the MutableState when your data changes, or
you can use an snapshot-backed collection such as SnapshotStateList or
SnapshotStateMap which will correctly cause a recomposition when their
contents are modified.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/test.kt:12:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val listProperty = mutableStateOf(list)
                       --------------
src/test/test.kt:14:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val setFunction = mutableStateOf(mutableSetOf(1))
                      --------------
src/test/test.kt:15:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val setProperty = mutableStateOf(set)
                      --------------
src/test/test.kt:17:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val mapFunction = mutableStateOf(mutableMapOf(1 to 1))
                      --------------
src/test/test.kt:18:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val mapProperty = mutableStateOf(map)
                      --------------
src/test/test.kt:20:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val collectionProperty = mutableStateOf(collection)
                             --------------
src/test/test.kt:28:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val listParameter = mutableStateOf(listParam)
                        --------------
src/test/test.kt:29:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val setParameter = mutableStateOf(setParam)
                       --------------
src/test/test.kt:30:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val mapParameter = mutableStateOf(mapParam)
                       --------------
src/test/test.kt:31:Warning: Creating a MutableState object with a
mutable collection type [MutableCollectionMutableState]
    val collectionProperty = mutableStateOf(collectionParam)
                             --------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package test

import androidx.compose.runtime.*

val list = mutableListOf(1)
val set = mutableSetOf(1)
val map = mutableMapOf(1 to 1)
val collection: MutableCollection<Int> = list

val listFunction = mutableStateOf(mutableListOf(1))
val listProperty = mutableStateOf(list)

val setFunction = mutableStateOf(mutableSetOf(1))
val setProperty = mutableStateOf(set)

val mapFunction = mutableStateOf(mutableMapOf(1 to 1))
val mapProperty = mutableStateOf(map)

val collectionProperty = mutableStateOf(collection)

fun test(
    listParam: MutableList<Int>,
    setParam: MutableSet<Int>,
    mapParam: MutableMap<Int, Int>,
    collectionParam: MutableCollection<Int>
) {
    val listParameter = mutableStateOf(listParam)
    val setParameter = mutableStateOf(setParam)
    val mapParameter = mutableStateOf(mapParam)
    val collectionProperty = mutableStateOf(collectionParam)
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/runtime/runtime-lint/src/test/java/androidx/compose/runtime/lint/MutableCollectionMutableStateDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `MutableCollectionMutableStateDetector.mutableCollection_stdlib`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=612128.

(##) Including

!!!
   This is not a built-in check. To include it, add the below dependency
   to your project.

```
// build.gradle.kts
implementation("androidx.compose.runtime:runtime-android:1.8.0-alpha07")

// build.gradle
implementation 'androidx.compose.runtime:runtime-android:1.8.0-alpha07'

// build.gradle.kts with version catalogs:
implementation(libs.runtime.android)

# libs.versions.toml
[versions]
runtime-android = "1.8.0-alpha07"
[libraries]
# For clarity and text wrapping purposes the following declaration is
# shown split up across lines, but in TOML it needs to be on a single
# line (see https://github.com/toml-lang/toml/issues/516) so adjust
# when pasting into libs.versions.toml:
runtime-android = {
    module = "androidx.compose.runtime:runtime-android",
    version.ref = "runtime-android"
}
```

1.8.0-alpha07 is the version this documentation was generated from;
there may be newer versions available.

NOTE: These lint checks are **also** made available separate from the main library.
You can also use `androidx.compose.runtime:runtime-lint:1.8.0-alpha07`.


[Additional details about androidx.compose.runtime:runtime-android](androidx_compose_runtime_runtime-android.md.html).
(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("MutableCollectionMutableState")
  fun method() {
     mutableStateOf(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("MutableCollectionMutableState")
  void method() {
     mutableStateOf(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection MutableCollectionMutableState
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="MutableCollectionMutableState" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'MutableCollectionMutableState'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore MutableCollectionMutableState ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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